1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and particularly relates to a method and apparatus for image forming capable of effectively detecting deterioration of developer.
2. Description of the Background Art
An image forming apparatus such as a copier, facsimile machine, printer, etc. generally includes an image bearing member such as a photoconductive element, and a developing device. Some background developing devices develop an electrostatic latent image formed on a surface of the image bearing member with two-component developer including toner and carrier.
When a developing device performs an image forming operation, the toner may constantly be consumed while the carrier may remain in the developing device. Even through fresh toner is supplied accordingly, the fresh toner may not be sufficiently agitated or mixed with the developer previously contained in the developing device. Some amount of the fresh toner cannot contact with magnetic carrier included in the developer in the above-described developing device and cannot substantially be charged or may be charged to an opposite polarity, causing problems such as image nonuniformity including fogging and contamination due to toner scattering.
The developing device may include an agitating member to agitate and mix the developer and the fresh toner, and uniformly distribute the fresh toner. However, repeated abrasion and collision of the toner and carrier in the developing device may cause deterioration of carrier such as contamination of a carrier particle and peeling of a coating layer of the carrier particle. This may gradually decrease a charging ability of the carrier, resulting in a substantial degradation of quality in an image.
The developer contained in a developer container of the developing device may be replaced on some regular basis before the problem related to image quality based on deterioration of carrier arises, but large labor and cost may be needed for such a replacement of the developer.
A trickle developing system may be employed to the developing device to supply developer including carrier and toner mixed therein or carrier and toner separately to a developer container of the developing device. The trickle developing system may supply fresh carrier as well as fresh toner, and discharge excess developer having degraded charging ability, or deteriorated developer, from the developer container to a collected developer container so that the deteriorated developer can be replaced and the degradation of the charging ability can be prevented.
The developing device with the trickle developing system supplies the amount of carrier in a constant ratio to the amount of consumed toner. However, a characteristic of carrier may significantly change depending on conditions for use of the image forming apparatus, generating a degradation of quality in an image when an amount of fresh carrier supplied to the developing device is too small to replace the deteriorated carrier, and resulting in a waste of carrier when an amount of fresh carrier supplied to the developing device is too large.
To prevent the degradation of charging ability, a small amount of fresh carrier may be supplied to the developing device in accordance with the frequency of the developing operations while the compensation of toner consumed in the developing operation is performed. This technique is proposed based on an idea that deterioration of carrier may be proportional to a period of time the developer container is operated or may be inversely proportional to an amount of toner consumed in the developing operation. The developing device may also detect the deterioration of carrier electrically.
It is not easy to directly detect the deterioration of carrier since a speed of deterioration of carrier depends on a temperature, humidity, a number of outputs, frequency of operations, etc. of the developer container. Because of frequent collision and abrasion with the fresh toner and the carrier previously contained in the developing device, some carriers allow the toner to adhere to the carriers when a larger amount of toner is consumed.
A toner density sensor is used to detect the deterioration of developer at a predetermined period of time after a beginning of using the developer, but is not provided with a specific standard to determine the deterioration of developer. Further, the decrease of a toner density or the number of printouts cannot determine the deterioration of developer. The determination of the deterioration of developer based on the number of printouts may ignore variations of the deterioration of carrier caused by differences of an image area ratio or an image pattern.
It is desirable that the developer may be supplied not based on a period of use time of the developer but based on a result obtained by the detection of the deterioration of carrier. However, no concrete mechanism of directly detecting the deterioration of carrier with high accuracy has been proposed. From a view point of cost reduction, previously proposed mechanisms may be more useful to detect the deterioration than inventing a brand new one.
The developer can automatically be replaced when a condition of the developer reaches a predetermined value, when the toner adheres to a conductive roller contacting the developer, or when the condition of the developer exceeds a predetermined electric value. However, the above-described changes in characteristic values cannot show whether toner or carrier causes the deterioration of developer. Since a replacement of developer may be performed by discharging the deteriorated carrier and supplying fresh carrier, a deterioration level of the carrier needs to be detected based on the entire characteristics of the developer.
A plurality of sensors including an optical sensor can be used to measure respective characteristics of the developer, such as magnetic permeability, toner density, an electric value, etc. However, for example, it is not obvious at which part of an apparatus the sensor measuring the electric value can be disposed. As described above, even though a detecting mechanism is provided, such detecting mechanism is not for detecting the deterioration of developer based on the characteristics of the entire developer.